your crakshaft is rotated by the pistons around 360 degres.
a spark has to ignite compressed fuel vapour in the combustion chamber at a certain time. you have to set up your sytem by syncronising both the crank and camshafts so this will happen. the component that delivers the spark is called the igntion coil which delivers a discontinous burst of sparks to the spark splug i.e kinda like bullets firing from a machine gun. a reasonable analogy is the machine gun attached to the front of german planes during WWII . the guns were attached top the front of the plane pointing straight ahead and were set up so as the bullets would fire through the propellars. well you have to set the cranshaft to a certain starting position so as the spark will occur at the correct time. for a compltete understanding you will have to learn about the ignition module(or breaker points, similar thing) and the distributor. the ignition module creates a break in the ignition ciruit determined by a senser in the distributor, which then allows the secondary coil in the igntion coil to be energised with a magnetic field. this is energy is then discharged down the HT leads to the plugs in the form of a pulse. maybe an importart point is thatr the IC would not discharge unless there is a break in the ciruit, hence breaker points. try
www.howstuffworks.com and go to the autos part.